The electrodes, i.e. plates of lead-acid storage batteries consist usually of a grid of lead alloy, which is filled with the active materials. The active materials are a paste consisting mainly of lead oxides, lead powder, sulphuric acid and are subjected to a partial drying, following which they are stored, in contact with each other, in order to allow chemical reactions to take place between the constituents of the paste, these reactions being essential for obtaining electrodes of the required quality.
The drying operation before storage is necessary for preventing the danger of sticking between electrodes. However, according to the results of modern research, the drying should be limited only to the surface of the electrodes, without any reduction of the water content inside the electrode. Otherwise, the fundamental chemical reactions during storage cannot be insured, resulting in a reduction both of the mechanical strength and the life of the electrodes.
According to a known, widely used method, the electrodes are conveyed through a tunnel oven kept at a temperature which insures, within a few minutes, the removal of water necessary to enable the electrodes to be stored in contact with each other without sticking. The application of this method showed that it is practically impossible to avoid the removal of moisture at great depths resulting both in a reduction of the water content inside the electrodes and in the formation of cracks. Furthermore, the removal of moisture is uneven, leading to the formation of areas of significantly different water content in the electrodes. Another disadvantage of the Brown method is that any irregularity in the feed rate creates intense variations of the drying conditions. Finally, the machinery necessary for the application of the method is bulky as the length of the tunnel is up to 7m.
Another known method (Greek Pat. No. 32591), according to which the electrodes are fed between one or more pairs of heated metal rolls, fulfills the requirement of even removal of surface humidity, but presents the following disadvantages:
The machinery for the application of the method requires complicated and tedious adjustment and has a tendency to frequent breakdowns.
The method requires a high uniformity in the thickness of the electrodes to be treated.
Any small irregularity in the thickness or flatness of any electrode results in jamming and creates a high amount of rejects.
If the thickness of the electrodes is higher than the thickness of the grids because of a surplus of active material (so-called "overpasting"), drying with the heated rolls method is practically impossible.